Retrofitting heritage buildings by strengthening or using seismic isolation

Many heritage buildings in the Mediterranean area include stone domes as a structural and architectural element. Present stage of these buildings often requires strengthening or retrofitting in order to increase their seismic resistance. Strengthening is possible by casting above existing dome a thin reinforced concrete shell with a support ring. It yields reduction of stresses and strains in the dome. This paper deals with examples of actual restoration and strengthening of three structures in Georgia, two of them damaged by an earthquake in 1991, (a temple in Nikortzminda and a synagogue in Oni, built in 11^th and 19^r century, respectively) and a mosque in Akhaltzikhe, built in 18th century. Retrofitting of these structures was aimed at preservation of initial geometry and appearance by creating composite (stone-reinforced concrete, or stone-shotcrete) structures, which were partially or fully hidden. Further improving of seismic response may be achieved by using hybrid seismic isolation decreasing the seismic forces and adding damping. A brief description of the design procedure for such cases is presented.